Automotive engineers continually strive to obtain complete combustion in hydrocarbon-fuel mixtures introduced into the combustion cylinders of reciprocating piston internal combustion engines. Engine cylinder blocks with their cylinder bores are carefully manufactured so that the bores are suitably round for receiving the pistons. Aluminum alloy pistons (for low weight and inertia) are cast or forged and machined to fit for high speed reciprocating motion in the cylinder bores. Generally, each piston has a head portion and a skirt portion. Circumferential grooves are machined near the top of each piston head to receive two compression rings and one or more oil retention rings. The respective rings are split at one point so that they can be placed around the piston head and in the grooves and then compressed inwardly when the assembly of piston and rings is fitted into a cylinder of an engine. Viewed from the working face of the piston head, the top and next lower compression rings serve to prevent hot combustion gas from blowing past the piston and into the lubrication oil containing crankcase. The bottom oil retention ring(s) scrape oil sprayed on the cylinder wall into a thin lubrication film.
Thus, the circumferential side of the piston head does not directly contact the surrounding cylinder bore. Rather, it is the piston rings that slide up and down on the lubricated cylinder wall. And there is an ever moving annular crevice between the top cylindrical land of the piston head, down to the top piston ring, and the cylinder bore. During the intake stroke of the piston, fuel can be caught in this moving crevice and subsequently not ignited when the air-fuel charge is burned. The energy content of this small portion of unburned fuel is not utilized in the power stroke of the engine. Instead the unburned fuel material is exhausted from the combustion chamber during the exhaust stroke of the piston, leading to unwanted emissions, particularly hydrocarbon emissions. Thus, there is a need for a piston head design that assures combustion of fuel trapped between the moving piston head and the cylinder wall of the engine block.